One known type of electrically driven power steering apparatus of a vehicle is that rotational output of an electric motor, which gives an assistive steering torque, is reduced speed by a gear unit, then is transmitted to a moving shaft (rack shaft, etc.) of a steering mechanism, and assists the manual power of a steering wheel so that the moving shaft is moved reciprocally in a predetermined range in order to steer the wheels. Here, a moving member impinges on the rack housing at the stroke end (end) of the moving member, and thereby an impact might occur in the transmission route of the steering apparatus. Such an impact might cause damage on the electric motor and the power transmission mechanism.
In order to relieve or eliminate such an impact, in known techniques, a torque limiter has been disposed at the rotational shaft of an electric motor, etc. (Japanese Patent Laid-Open Publication No. 9-84300, or Japanese Patent Laid-Open Publication No. 9-221045).
However, in recent years, although there are demands for restraining the inertia of an electric motor for the reason of improvement of steering feeling, and the like, there is a problem in that, with the structure of incorporating a torque limiter in the electric motor, the inertial of rotational shaft becomes large, and the electric motor cannot be miniaturized. Also, with a structure in which a friction member is used for a function of the torque limiter, there has been a problem in that the torque limiter has a possibility of deteriorating from its initial level of performance as a result of wear-out of the friction member during its use over time.
To overcome such problems, in Japanese Patent Laid-Open Publication No. 8-11728, a disc spring is attached to the contact portion of the rack end of the rack shaft, which is a moving shaft, with the housing in order to absorb the impact of the stroke end. In such a structure, the problems as described above will not arise. However, when the disc spring is caused to function as an absorbing material in a limited space, an induced stress becomes high, and thus the load imposed on the disc spring might be high.